Flat steel products are typically rolled products such as steel strips or sheets, and blanks and plates produced therefrom.
All figures relating to contents of the steel compositions specified in the present application are based on weight, unless explicitly stated otherwise. All otherwise indeterminate percentages in connection with a steel alloy should therefore be understood as figures in “% by weight”.
High-strength sheet metal strips are of growing significance since an important role is nowadays played not only by technical performance but also by resource efficiency and climate protection. The reduction in the intrinsic weight of a steel construction can be achieved by the enhancement of the strength properties.
As well as high strength, high-strength steel strips and sheets have to meet high demands on toughness properties and brittle fracture resistance, on cold forming characteristics and on suitability for welding.
Conventional production of the ultrahigh-strength steels consists of rolling, hardening and tempering. In the production of high-strength steel products having a minimum yield strength of 900 MPa, slabs are first cast from a steel melt of suitable composition. The slabs are then hot-rolled to give sheets or strips, which are then cooled under air. The flat steel products obtained have a ferritic-pearlitic microstructure. In order to establish the desired martensitic-bainitic microstructure, the flat steel products are then heated to a temperature above the Ac3 temperature and quenched with water.
To adjust the toughness, in the conventional procedure, the hardening microstructure has to be subjected to a tempering treatment in a further step. The conventional production process thus entails several stages in order to attain the required mechanical properties of the flat steel product to be produced. The large number of operating steps associated with the conventional mode of production leads to comparably high production costs. At the same time, in spite of the complex process sequence, the toughness properties and surface quality of the high-strength flat steel products produced by the conventional route are frequently nonoptimal.
EP 1 669 470 A1 discloses a hot-rolled steel strip having a steel composition comprising (in % by weight) 0.01%-0.2% by weight of C, 0.01%-2% Si, 0.1%-2% Mn, up to 0.1% P, up to 0.03% S, 0.001%-0.1% Al, up to 0.01% N and, as the remainder, Fe and unavoidable impurities. This flat steel product has an essentially homogeneously and continuously cooled microstructure having a mean grain size of 8 μm to 30 μm. In order to achieve this, a slab having the above-specified composition is rough-rolled. The rough-rolled slab obtained is then finally hot-rolled at a hot rolling end temperature at least 50° C. above the Ar3 temperature of the steel to give a hot strip. Subsequently, the finally hot-rolled hot strip, after a delay of at least 0.5 second, is cooled at a cooling rate of at least 80° C./sec from the Ar3 temperature to a coiling temperature of less than 500° C. and finally coiled to a coil.
WO 03/031669 A1 additionally discloses a high-strength thin steel sheet which is deep-drawable and at the same time has excellent shape retention. Furthermore, this publication describes a method of producing such a flat steel product. The steel sheet in question is notable for a particular ratio of x-ray intensities of particular crystallographic orientations and has a particular roughness Ra and a particular coefficient of friction of the steel sheet surface at up to 200° C., and has a lubricant effect. For production of such flat steel products, a hot strip of suitable composition is produced by hot rolling with a total reduction ratio of at least 25% at a temperature within a range between the Ar3 temperature and the Ar3 temperature +100° C. In all flat steel products produced by this method, ferrite is present in the microstructure.